The in-mold forming method is known as a method for fabricating, by integral molding, a synthetic resin formed product originally attached with a label. In this method, a label is first placed on the inner wall of the metal mold, and then a forming resin such as polyethylene and polypropylene is directly fed into the metal mold to obtain a formed product by injection molding, hollow molding or in-mold bead foaming. For the case of attaching a label onto polypropylene or polyethylene sheet, the attachment can be effected based on the same principle by a method such as that promoting forming based on differential pressure under heating of the sheet (vacuum molding, differential pressure molding).
A variety of transparent labels have been proposed as available for such in-mold forming. An exemplary transparent label is that obtained by coating on a substrate layer a solution of low-melting-point olefin resins such as ethylene-vinyl acetate copolymer using a gravure coater or the like and by successive drying; where the substrate layer comprises a non-stretched extruded film made of crystalline polypropylene or high-density polyethylene, or a transparent film formed by the calendar process. There are also proposed films such that fabricated by stacking a low-melting-point olefin base resin film on one surface of the substrate layer using an adhesive or by direct extrusion lamination.
This attached label, however, is less attractive in that lacking gross and quality sense since the film composing the substrate layer is of non-stretched type. Another problem is that the film may easily be stretched due to tensile force when it is subjected to gravure printing in a wound-up state, which may result in misalignment of patterns and prevent high-definition printing. Still another problem is that crimping of the film due to lack of its stiffness, which is likely to occur when the label cut into a predetermined size is attached to the inner wall of the metal mold using a robot inserter, so that high-speed attachment of the film will result into failure. Moreover, a label coated or laminated with an adhesive layer requires excessive process steps and, as a result, higher production cost.
On the other hand, as a transparent label for overcoming the foregoing drawbacks, proposed is a label which is fabricated by laminating low-melting-point olefin resin such as a low-density polyethylene or ethylene-vinyl acetate copolymer on a substrate layer comprising a sheet of longitudinally stretched crystalline polypropylene, and by transversely stretching the laminate using a tenter oven. Decorating polyethylene using such label by hollow molding, however, requires a parison temperature of 180.degree. C. or above and a metal mold cooling temperature or 30.degree. C. or above. As for hollow molding of polypropylene, a satisfactory product also cannot be obtained unless the parison temperature is raised as high as 230.degree. C. or above and the metal mold cooling temperature is as high as 30.degree. C. or above. Conditions out of the above ranges will result in generation of blisters which lowers the adhesive strength of the label and prevents practical products from being produced.
To ensure high-speed and low-cost hollow molding of polyethylene (m.p. 108-135.degree. C.) containers, it is desirable in general to lower the parison temperature as possible and shorten the cooling period thereby to shorten the forming cycle. Whereas, treating a transparent label having a substrate layer made of a biaxially stretched film or non-stretched layer film at the above required temperatures will weaken the adhesive strength of the label to cause blisters, and will fail in obtaining practical products.